As far as I can tell the boto3 client for ECS does not support the ability to trim down the response document.

I think if I wanted to speed up execution I would have a separate service to query and then cache the results. I would then query the cache instead of working directly with real time data. I'm not sure if your problem can deal with slightly stale data.

My questions for you:

How long is your AWS Lambda execution time right now?

How certain are you that waiting for the ECS service descriptions is the slowest part of the current implementation?

How often does your AWS Lambda run?

If you could have instant ECS service descriptions, how much would you really save?

Is this really the best cost saving problem you could be working on?

I'm naive of why you want to speed up execution times. If you want to save on cost, I personally wouldn't bother. Engineering a robust solution to speed up execution time is likely going to cost more than what your Lambda bill will be.

If it's to speed up a long pipeline of dependent tasks, I think I would try engineering some sort of caching service instead of working directly with AWS API responses.